Method for controlling non-aqueous electrolyte secondary battery

ABSTRACT

A method for controlling a non-aqueous electrolyte secondary battery that includes connecting two non-aqueous electrolyte secondary batteries in series and setting the discharge cutoff voltage to 3.4 V to 4.6 V.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International applicationNo. PCT/JP2015/069175, filed Jul. 2, 2015, which claims priority toJapanese Patent Application No. 2014-175393, filed Aug. 29, 2014, theentire contents of each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery such as a lithium ion secondary battery, and more particularly,to a method for controlling a non-aqueous electrolyte secondary battery,for suppressing cycle characteristic degradation in the case of usingtwo non-aqueous electrolyte secondary batteries connected in series.

BACKGROUND OF THE INVENTION

In recent years, the reduction in size and weight for cellular phones,laptop computers, and the like has progressed rapidly, and batteries aspower sources for driving the phones, the computers, and the like havebeen required to have higher capacities. Further, under suchcircumstances, non-aqueous electrolyte secondary batteries typified bylithium ion secondary batteries have been widely used as power sources.

Now, high-energy density non-aqueous electrolyte secondary batteriestypified by lithium ion secondary batteries and the like as describedabove are generally manufactured by such a method as described below.

First, rolled sheet-like current collector foil (e.g., aluminum foil,copper foil) is run through a die coater, a comma coater, or the like toapply an active material (e.g., a lithium composite oxide, carbon) ontothe current collector foil, thereby preparing sheet-like electrodes.

Then, the sheet-like electrodes are stacked or wound with separatorsinterposed therebetween for preventing short circuits due to contactbetween the electrodes, thereby preparing electrode groups.

Thereafter, aluminum tabs or nickel tabs to serve as external terminalelectrodes are welded to the electrodes by ultrasonic welding, so as tomake electrical connections to the electrodes.

Then, elements including the thus prepared electrode groups are put inexterior packages such as aluminum cans or aluminum laminate films, andthe packages are sealed after injecting an electrolytic solution.

Thus, non-aqueous electrolyte secondary batteries are prepared.

Incidentally, in recent years, as typified by batteries of hybridvehicles, non-aqueous electrolyte secondary batteries (electric storagedevices) have strongly required to achieve higher reliability for cyclecharacteristics and the like, and charge/discharge rate characteristicsimproved by lowered resistance.

As such non-aqueous electrolyte secondary batteries, non-aqueouselectrolyte secondary batteries have been studied which uselithium-titanium oxides for negative electrode active materials.Lithium-titanium oxides for negative electrode active materials are lesslikely to undergo a change in crystal lattice volume with charge anddischarge, and thus less likely to be degraded by expansion andcontraction of the crystal structure, and the reaction between thenegative electrode and an electrolytic solution is inhibited because thehigh potential for storage and release of lithium ions is +1.55 V on thebasis of Li/Li+. The oxides are known to have high reliability for cyclecharacteristics and the like, as compared with when carbon such asgraphite is used for negative electrode active materials.

Further, as a technique for improving resistance to overdischarge andresistance to overcharge at high temperatures of 60 to 80° C., PatentDocument 1 proposes a non-aqueous electrolyte lithium secondary batterywhich has reliability improved by making a negative electrode capacity(mAh) lower than a positive electrode capacity (mAh).

However, in the case of using non-aqueous electrolyte secondarybatteries proposed conventionally, there is the problem of failing todevelop cycle characteristics of single cells even in the case of adischarge cutoff voltage of, for example, 2.0 V (that is, in the case ofa discharge cutoff voltage of 1.0 V per single cell) when twonon-aqueous electrolyte secondary batteries are connected in series andused, even though favorable cycle characteristics are achieved in thecase of a discharge cutoff voltage (for example, the discharge cutoffvoltage of 1.0 V in Patent Document 1) per non-aqueous electrolytesecondary battery (single cell).

More specifically, even when charge-discharge cycles are performed underthe same condition, in the case of using two non-aqueous electrolytesecondary batteries connected in series, there is the problem of losingthe voltage balance during discharge, thereby resulting in cyclecharacteristics degraded dramatically as compared with cyclecharacteristics of single cells.

In addition, in order to respond to such a lost voltage balance, it isalso common to provide balance circuits, which have the problem ofcausing an increase in cost and are complex in structure.

Patent Document 1: Japanese Patent Application Laid-Open No. 10-69922

SUMMARY OF THE INVENTION

The present invention is intended to solve the problems mentioned above,an object of the invention is to provide a method for controlling anon-aqueous electrolyte secondary battery, which is capable ofsuppressing cycle characteristic degradation in the case of using twonon-aqueous electrolyte secondary batteries connected in series.

In order to solve the problems, the present invention provides a methodfor controlling a non-aqueous electrolyte secondary battery. Thenon-aqueous electrolyte secondary battery includes an exterior material;a positive electrode housed in the exterior material, the positiveelectrode including a lithium-transition metal oxide that has a layeredcrystal structure; a negative electrode housed in the exterior material,the negative electrode including a spinel-type lithium-titaniumcomposite oxide; and a non-aqueous electrolyte packed in the exteriormaterial. The positive electrode and the negative electrode meet acondition of the following formula (1):

1.0>X   (1)

X represents an actual electric capacity ratio denoted by (B/A), Arepresents an actual electric capacity (mAh) at 25° C. per area 1 cm² ofthe positive electrode, and B represents an actual electric capacity(mAh) at 25° C. per area 1 cm² of the negative electrode. A dischargecutoff voltage is set to 3.4 V to 4.6 V when two non-aqueous electrolytesecondary batteries are connected in series.

In accordance with the method for controlling a non-aqueous electrolytesecondary battery according to the present invention, the dischargecutoff voltage is set to be 3.4 V to 4.6 V when using two non-aqueouselectrolyte secondary batteries connected in series, and cyclecharacteristics of the two-series cell can be thus improvedsignificantly.

In addition, the need for balance circuits is eliminated, thereby makingit possible to reduce the number of components significantly.

More specifically, in the case of using two non-aqueous electrolytesecondary batteries connected in series, each basically composed of: anegative electrode containing, as its main constituent, a spinel-typelithium-titanium composite oxide; a positive electrode that has a higherpotential than the spinel-type lithium-titanium composite oxide; and anorganic electrolytic solution, where the actual electric capacity perunit area of the negative electrode is made lower than the actualelectric capacity per unit are of the positive electrode, the dischargecutoff voltage is set to be 3.4 V or higher, thereby making it possiblefor the relation between the discharge potentials of one non-aqueouselectrolyte secondary battery and the other non-aqueous electrolytesecondary battery to fall within the range of 2.3 V:1.1 V to 1.7 V:1.7V, and making it possible to suppress, even when one non-aqueouselectrolyte secondary battery and the other of the two non-aqueouselectrolyte secondary batteries connected in series lose the capacitybalance therebetween, the phenomenon of causing overdischargedegradation and causing the overdischarge degradation to cause furtheroverdischarge degradation.

Details will be described in the following embodiment.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating a battery element ofa non-aqueous electrolyte secondary battery to which a control methodaccording to an embodiment of the present invention is applied, that is,a battery element including a positive electrode, a negative electrode,and a separator.

FIG. 2 is a perspective view illustrating the appearance configurationof a non-aqueous electrolyte secondary battery to which a control methodaccording to an embodiment of the present invention is applied.

FIG. 3 is a diagram showing the result of a cycle test 1 carried outwith a single cell for a non-aqueous electrolyte secondary battery.

FIG. 4 is a diagram showing the result of a cycle test 2 carried outwith a single cell for a non-aqueous electrolyte secondary battery.

FIG. 5 is a diagram showing the result of a cycle test 3 carried outwith a single cell for a non-aqueous electrolyte secondary battery.

FIG. 6 is a diagram showing the result of a cycle test 4 carried outwith a single cell for a non-aqueous electrolyte secondary battery.

FIG. 7 is a diagram showing the result of a cycle test 5 carried outwith a single cell for a non-aqueous electrolyte secondary battery.

FIG. 8 is a diagram showing the result of a cycle test 6 carried outwith a single cell for a non-aqueous electrolyte secondary battery.

FIG. 9 is a diagram showing the course of a 5.0 V

3.0 V cycle test at 25° C. for a two-series non-aqueous electrolytesecondary battery C1 that has favorable cycle characteristics and atwo-series non-aqueous electrolyte secondary battery C2 that has cyclecharacteristics significantly degraded.

FIG. 10 is a diagram showing the results of checking: (a) the voltagebehavior in the case of discharging the whole two-series non-aqueouselectrolyte secondary battery C2 (two-series cell); and; and (b) thevoltage behaviors (voltage balance) in the case of discharging each ofnon-aqueous electrolyte secondary batteries C2 a and C2 b constitutingthe two-series non-aqueous electrolyte secondary battery C2, for thetwo-series non-aqueous electrolyte secondary battery C2 after theimplementation of 772 cycles, with cycle characteristics significantlydegraded, as shown in FIG. 9.

FIG. 11 is a diagram showing a model for a discharge behavior of asingle cell for a non-aqueous electrolyte secondary battery to which thecontrol method according to the present invention is applied.

DETAILED DESCRIPTION OF THE INVENTION

Prior to providing a specific embodiment of the invention, a summary ofa configuration according to the present invention will be presentedfirst.

The non-aqueous electrolyte secondary battery to which a control methodaccording to the present invention is applied, includes a positiveelectrode, a negative electrode and an electrolytic solution. Thepositive electrode can include aluminum foil as a positive electrodecurrent collector, and a combination material layer including alithium-transition metal oxide that has a layered crystal structure as apositive electrode active material layer on the aluminum foil. Thelithium-transition metal oxide that has a layered crystal structure canbe, for example, a lithium composite oxide such as LiCoO₂ orLiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂.

The negative electrode can include aluminum foil as a negative electrodecurrent collector layer, and a combination material layer including alithium-titanium oxide that has a spinel-type crystal structure as anegative electrode active material layer on the aluminum foil.Furthermore, a separator layer is interposed between the opposedpositive electrode and negative electrode, thereby preventing shortcircuits due to contact between the electrodes.

The electrolytic solution can be, for example, an electrolytic solutionwhere one or more electrolytes selected from LiPF₆, LiBF₄, and LiTFSIare dissolved in one solvent selected from dimethyl carbonate, diethylcarbonate, methyl ethyl carbonate, propylene carbonate, andacetonitrile, or a mixed organic solvent of multiple solvents selectedtherefrom.

Among these electrolytic solutions, typical electrolytic solutionsinclude 1 mol/L of LiPF₆ dissolved in a mixed solvent of propylenecarbonate.

In addition, electrolytic solutions and the like can be also used wherethe organic solvents and electrolyte salts mentioned above are dissolvedin one or more ionic liquids selected from 1-ethyl-3-methylimidazoliumtetrafluoroborate and1-ethyl-3methylimidazoliumbis(trifluoromethanesulfonyl)imide assolvents.

The charge cutoff voltage is 5.50 V, preferably 5.20 V, and morepreferably 5.00 V, whereas the discharge cutoff voltage is desirably3.40 V.

It is to be noted that according to the present invention, the dischargecutoff voltage has an upper limit of 4.6 V, due to the fact that thedischarge voltage required to produce a substantial capacity is 4.6 V(2.3 V per single cell (see FIG. 11)) in the two-series non-aqueouselectrolyte secondary battery as shown in FIG. 11.

Features of the present invention will be described in detail below withreference to an embodiment of the present invention.

Embodiment

(1) Preparation of Positive Electrode

A lithium-cobalt composite oxide (LCO) represented by the compositionformula LiCoO₂ as a positive electrode active material, carbon as aconductive agent, and polyvinylidene fluoride (PVDF) as a binder werecombined to meet 90:7:3 in ratio by weight, and kneaded withN-methyl-2-pyrrolidone (NMP), thereby preparing slurry. This slurry wasapplied to aluminum foil as a current collector, so as to reachpredetermined weight, dried, and then subjected to rolling by rollpress, and to punching into 3 cm², thereby preparing a positiveelectrode.

The thickness of the positive electrode was adjusted by roll press suchthat the positive electrode was 3.3 g/cm³ in filling density.

The weight of the positive electrode (layer) was 8.8 mg/cm² on one side.

(2) Preparation of Negative Electrode

A spinel-type lithium-titanium composite oxide represented by Li₄Ti₅O₁₂as a negative electrode active material and PVDF as a binder werecombined to meet 95:5 in ratio by weight, and kneaded with NMP, therebypreparing slurry.

This slurry was applied to aluminum foil as a current collector, so asto reach predetermined weight, dried, and then subjected to rolling byroll press, and to punching into 3 cm², thereby preparing a negativeelectrode.

The thickness of the negative electrode layer was adjusted by roll presssuch that the negative electrode layer was all 2.0 g/cm³ in fillingdensity.

The weight of the negative electrode layer was 10.6 mg/cm² on one side.

(3) Preparation of Non-Aqueous Electrolytic Solution

A non-aqueous electrolytic solution was prepared by dissolving 1 mol/lof LiPF₆ in a mixed solvent of propylene carbonate.

(4) Preparation of Battery

As shown in FIG. 1, a positive electrode 11 and a negative electrode 12prepared by the methods mentioned above were provided with lead tabs 14,15. A porous separator 13 (air permeability: 10 sec. 100 cc) composed ofa polyamideimide was interposed and laminated between the positiveelectrode 11 and the negative electrode 12, thereby preparing a batteryelement 10.

Then, after attaching sealants 16 and 17 respectively to the lead tabs14 and 15, the battery element (laminated body) 10 was housed in anouter package material 18 composed of a laminate film including analuminum layer as an intermediate layer between resin layers, as shownin FIG. 2. Thereafter, a non-aqueous electrolytic solution prepared bythe method mentioned above was injected into the outer package material18, and an opening of the outer package material 18 was then sealed,thereby preparing a non-aqueous electrolyte secondary battery 20. It isto be noted that the outer package material 18 is not to be consideredlimited to any sac-shaped material that uses a laminate film asdescribed above, but it is possible to use various forms that can sealthe battery element along with the non-aqueous electrolytic solution,and it is also possible to use, for example, a can-shaped material.

It is to be noted that this non-aqueous electrolyte secondary battery 20is configured such that the actual electric capacity per unit area ofthe negative electrode 12 is lower than the actual electric capacity perunit area of the positive electrode 11, and the battery is configuredsuch that the positive electrode and the negative electrode meet therequirement expressed by the following formula (1):

1.0>X   (1)

(X represents an actual electric capacity ratio denoted by (B/A), Arepresents an actual electric capacity (mAh) at 25° C. per area 1 cm² ofthe positive electrode, and B represents an actual electric capacity(mAh) at 25° C. per area 1 cm² of the negative electrode)

(5) High-Temperature Rapid Charge-Discharge Cycle Test

(5-1) A two-series non-aqueous electrolyte secondary battery containingtwo series-connected non-aqueous electrolyte secondary batteriesprepared in the way described above was subjected to a high-temperaturerapid charge-discharge cycle test. For the high-temperature rapidcharge-discharge cycle test, first, under an atmosphere at a temperatureof 70° C., the battery was subjected to constant-current charge at acurrent of 50 mA until the voltage reached 5.0 V, and after reaching 5.0V, subjected to constant-voltage charge until the current reached 0.2mA.

Thereafter, with a current of 50 mA, the battery was subjected toconstant-current discharge:

(a) until the voltage (discharge cutoff voltage) reached 3.0 V as acondition 1;

(b) until the voltage (discharge cutoff voltage) reached 3.2 V as acondition 2;

(c) until the voltage (discharge cutoff voltage) reached 3.4 V as acondition 3;

(d) until the voltage (discharge cutoff voltage) reached 3.6 V as acondition 4; or

(e) until the voltage (discharge cutoff voltage) reached 3.8 V as acondition 5, as one cycle.

It is to be noted that for the sake of safety, the cycle measurement wasstopped when the capacity retention rate fell down below 50% (in thecase of the condition 1 with the discharge cutoff voltage of 3.0 V).

Then, this cycle was performed 2000 cycles, thereby checking thecapacity retention rate after the 2000 cycles in the high-temperaturerapid charge-discharge cycle test.

It is to be noted that the capacity retention rate (%) was obtained fromthe following formula (2).

(Discharge Capacity in 2000-th Cycle/Discharge Capacity in FirstCycle)×100   (2)

Table 1 shows the capacity retention rates after the 2000 cycles in thecase of the test carried out under the conditions 1 to 5 describedabove, for the two-series non-aqueous electrolyte secondary battery.

TABLE 1 Discharge cutoff Capacity retention voltage (V) rate after 2000cycles Condition 1 3.0 — Condition 2 3.2  52% Condition 3 3.4 100%Condition 4 3.6 100% Condition 5 3.8 100%

Among the respective conditions described above, under the conditions 3to 5 with the discharge cutoff voltages of 3.4 V, 3.6 V, and 3.8 V, thecapacity retention rates were kept at 100%.

On the other hand, in the case of the condition 2 with the dischargecutoff voltage of 3.2 V, the capacity retention rate was decreased downto 52%. In addition, in the case of the condition 1 with the dischargecutoff voltage of 3.2 V, the capacity retention rate fell down below50%.

From the foregoing results, it is determined that in the case of thetwo-series non-aqueous electrolyte secondary battery described above,the cutoff voltage of 3.4 V or higher can achieve the capacity retentionrate of 100% in the cycle test of the 2000 cycles at 70° C.

(5-2) In addition, for the non-aqueous electrolyte secondary battery inthe way described above, the single cell was subjected to:

1) a cycle test 1 of repeatedly charging the cell until the voltagereached 2.5 V and discharging the cell until the voltage reached 0.5 V;

2) a cycle test 2 of repeatedly charging the cell until the voltagereached 2.5 V and discharging the cell until the voltage reached 1.0 V;

3) a cycle test 3 of repeatedly charging the cell until the voltagereached 2.5 V and discharging the cell until the voltage reached 1.25 V;

4) a cycle test 4 of repeatedly charging the cell until the voltagereached 2.5 V and discharging the cell until the voltage reached 1.5 V;

5) a cycle test 5 of repeatedly charging the cell until the voltagereached 2.55 V and discharging the cell until the voltage reached 1.5 V;and

6) a cycle test 6 of repeatedly charging the cell until the voltagereached 2.6 V and discharging the cell until the voltage reached 1.5 V;

FIGS. 3 to 8 show the results, or the results of the cycle tests 1 to 6.

From FIGS. 3 to 8, it is determined that the non-aqueous electrolytesecondary battery configured as described above has, in the case of thesingle cell, favorable cycle characteristics when the discharge cutoffvoltage exceeds 1.0 V, and cycle characteristic degradation issignificantly affected by the low discharge voltage, rather than thevalue of the charge voltage which is generally likely to affect cyclecharacteristics.

It is believed that the low discharge voltage significantly affectscycle characteristic degradation, because an unstable Li₄-xTi₅O₁₂structure has high activity in a mixed state of trivalent titanium andtetravalent titanium, in particular, in a state with more tetravalenttitanium at the end of the discharge, and thus react with theelectrolytic solution, thereby leading to electrode destructionassociated with gas generation.

In particular, when the positive electrode and the negative electrodemeet the relation of positive electrode>negative electrode in terms ofmagnitude of actual electric capacity (mAh), the voltage of the positiveelectrode drops at the end of the discharge, but when the actualelectric capacity (mAh) of the negative electrode is made lower than theactual electric capacity (mAh) of the positive electrode, the negativeelectrode is turned to a more unstable Li₄-xTi₅O₁₂ structure with highactivity at the end of the discharge, thereby leading to cyclecharacteristic degradation.

On the other hand, for the two-series non-aqueous electrolyte secondarybattery (sample), a cycle test was carried out under the followingcondition.

Charge: 50 mA/5 V to 0.5 mA

Pause: 30 sec

Discharge: 50 mA to 3 V

Pause: 30 sec

Temperature: 25° C.

As a result, it has been confirmed that some of multiple samples(two-series non-aqueous electrolyte secondary batteries) have favorablecycle characteristics, whereas the others have cycle characteristicssignificantly degraded.

FIG. 9 is a diagram showing the course of a 5.0 V

3.0 V cycle test at 25° C. for a two-series non-aqueous electrolytesecondary battery C1 that has favorable cycle characteristics and atwo-series non-aqueous electrolyte secondary battery C2 that has cyclecharacteristics significantly degraded.

Further, the two-series non-aqueous electrolyte secondary battery C2after the implementation of 772 cycles, with cycle characteristicssignificantly degraded, as shown in FIG. 9, was checked for:

(a) the voltage behavior in the case of discharging the whole two-seriesnon-aqueous electrolyte secondary battery C2 (two-series cell); and

(b) the voltage behaviors (voltage balance) in the case of dischargingeach of non-aqueous electrolyte secondary batteries C2 a and C2 bconstituting the two-series non-aqueous electrolyte secondary batteryC2.

It is to be noted that the test condition is the same as the conditionin the case of the cycle test for the two-series non-aqueous electrolytesecondary battery (sample) as mentioned above. The results are shown inFIG. 10.

As shown in FIG. 10, the two-series non-aqueous electrolyte secondarybattery C2 (two-series cell) underwent a voltage drop to 3 V. Incontrast, it has been confirmed that one non-aqueous electrolytesecondary battery C2 a (single cell) of the two non-aqueous electrolytesecondary batteries C2 a and C2 b connected in series underwent avoltage drop to approximately 2 V, whereas the other non-aqueouselectrolyte secondary battery C2 b (single cell) underwent a voltagedrop to around 1 V, thereby resulting in the discharge cut off.

Further, Table 2 shows the discharge capacities (mAh) of the two-seriesnon-aqueous electrolyte secondary battery C2, and of one non-aqueouselectrolyte secondary battery C2 a (single cell) and the othernon-aqueous electrolyte secondary battery C2 b (single cell)constituting the two-series non-aqueous electrolyte secondary battery C2 .

TABLE 2 One non-aqueous The other non- Two-series non- electrolyteaqueous electrolyte aqueous electrolyte secondary battery secondarybattery secondary battery (single cell) C2a (single cell) C2b(two-series cell) C2 Discharge 10.90 6.48 6.45 capacity (mAh)

As shown in Table 2, it has been confirmed that even when onenon-aqueous electrolyte secondary battery C2 a has a high dischargecapacity, while the other non-aqueous electrolyte secondary battery C2 bhas a low discharge capacity, the two-series non-aqueous electrolytesecondary battery C2 of the two connected in series has a reduceddischarge capacity.

From this result, it has been determined that in the case of thetwo-series non-aqueous electrolyte secondary battery having the twonon-aqueous electrolyte secondary batteries connected in series asdescribed above, the single cells have favorable cycle characteristicsto the discharge cutoff voltage in excess of 1.0 V, while there is atendency to result in significantly degraded cycle characteristics inspite of the cutoff voltage per single cell equivalent to 1.5 V (thedischarge cutoff voltage of 3 V in the two-series non-aqueouselectrolyte secondary battery) in the case of using the two connected inseries.

Therefore, in order to prevent such an event, a model for a dischargebehavior at a single cell was considered for a non-aqueous electrolytesecondary battery to which the control method according to the presentinvention is applied.

It is to be noted that FIG. 11 is a diagram showing a model for adischarge behavior at a single cell for a non-aqueous electrolytesecondary battery to which the control method according to the presentinvention is applied.

As a result of the consideration, events such as:

[1] as shown in FIG. 11, the fact that there is a plateau region (aregion with small changes in potential) around 2.3 V;

[2] in addition, as can be seen from FIG. 11, the fact that there is asudden drop in voltage even with a “slight difference in capacity” atthe end of the discharge; and

[3] furthermore, as can be seen from FIG. 11, the fact that to there isan instant drop in voltage to a voltage around 2.3 V in the case ofdischarge from a state of full charge have been confirmed.

In addition, Table 3 shows the fluctuation ranges of the dischargevoltages for one non-aqueous electrolyte secondary battery C2 a and theother non-aqueous electrolyte secondary battery C2 b in the case of thedischarge cutoff voltage set to be 3.4 V for the two-series non-aqueouselectrolyte secondary battery configured according to this embodiment,and Table 4 shows the fluctuation ranges of the discharge ranges for thefluctuation regions of the discharge voltages for one non-aqueouselectrolyte secondary battery C2 a and the other non-aqueous electrolytesecondary battery C2 b in the case of the discharge cutoff voltage setto be 3.0 V therefor.

TABLE 3 Discharge Voltages of Batteries C2a, C2b in the case ofDischarge Cutoff Voltage of 3.4 V Battery 2.3 2.2 2.1 2.0 1.9 1.8 1.7C2a Battery 1.1 1.2 1.3 1.4 1.5 1.6 1.7 C2b

TABLE 4 Discharge Voltages of Batteries C2a, C2b in the case ofDischarge Cutoff Voltage of 3.0 V Battery 2.3 2.2 2.1 2.0 1.9 1.8 1.71.6 1.5 C2a Battery 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 C2b

As shown in Table 3, in the case of the two-series non-aqueouselectrolyte secondary battery configured according to this embodiment,the discharge potentials for one non-aqueous electrolyte secondarybattery and the other non-aqueous electrolyte secondary battery fallwithin the range of 2.3 V:1.1 V to 1.7 V:1.7 V in the case of thedischarge cutoff voltage set to be 3.4 V.

On the other hand, as shown in Table 4, it is determined that even withthe two-series non-aqueous electrolyte secondary battery configuredaccording to this embodiment, the discharge potentials for onenon-aqueous electrolyte secondary battery and the other non-aqueouselectrolyte secondary battery unfavorably vary from 2.3 V:0.7 V even to1.5 V:1.5 V, and may be lower than 1.0 V in the case of the dischargecutoff voltage set to be 3.0 V.

As a result of the foregoing consideration, the conclusion is drawn asdescribed below.

In the two-series non-aqueous electrolyte secondary battery having thetwo non-aqueous electrolyte secondary batteries (non-aqueous electrolytesecondary batteries to which the control method according to the presentinvention is applied) connected in series, the discharge voltages varydifferently with a “slight difference in capacity” at the end of thedischarge, even when one non-aqueous electrolyte secondary battery andthe other of the two non-aqueous electrolyte secondary batteries arealmost equal in discharge voltage (for example, 1.8 V) (see thedescription in the foregoing section [2]).

However, as in the present invention, cycles with a lower limit voltageof 3.4 V cause the relation between the discharge potentials of the twonon-aqueous electrolyte secondary batteries to fall within the range of2.3 V:1.1 V to 1.7 V:1.7 V as shown in Table 3, because of an instantdrop in voltage in the case of discharge from a state of full charge,for example, even when the discharge cutoff voltage is decreased foreither one of the two non-aqueous electrolyte secondary batteries, andnever cause the potentials to be 1.0 V or lower for either of thenon-aqueous electrolyte secondary batteries (the voltage per single cellis never 1.0 V or lover) (see the description in the foregoing section[3]).

Further, it is also clear that the single cells have favorable cyclecharacteristics to the discharge cutoff voltage in excess of 1.0 V, fromthe foregoing results of the cycle tests for the single cell as shown inFIGS. 3 to 8.

Accordingly, in the two-series non-aqueous electrolyte secondary batteryhaving the two non-aqueous electrolyte secondary batteries connected inseries to which the control method according to the present invention isapplied, the discharge cutoff voltage is set to fall within the range of3.4 V to 4.6 V, thereby making it possible to prevent, even when onenon-aqueous electrolyte secondary battery and the other lose thecapacity balance therebetween, the phenomenon of causing overdischargedegradation and causing the overdischarge degradation to cause furtheroverdischarge degradation, and thus improve cycle characteristics of thetwo-series non-aqueous electrolyte secondary battery dramatically.

More specifically, in the case of using the two-series non-aqueouselectrolyte secondary battery having the two non-aqueous electrolytesecondary batteries connected in series, each basically composed of: anegative electrode containing, as its main constituent, a spinel-typelithium-titanium composite oxide; a positive electrode that has a higherpotential than the spinel-type lithium-titanium composite oxide; and anorganic electrolytic solution, where the electric capacity of thenegative electrode is made lower than the electric capacity of arechargeable region of the positive electrode, the discharge cutoffvoltage is set to fall within the range of 3.4 V or more and 4.6 V orless, thereby making it possible to prevent, even when one non-aqueouselectrolyte secondary battery and the other, the two lose the capacitybalance therebetween, the phenomenon of causing overdischargedegradation and causing the overdischarge degradation to cause furtheroverdischarge degradation, and thus improve cycle characteristics of thetwo-series non-aqueous electrolyte secondary battery substantially.

In addition, the need of balance circuits which have been requiredconventionally is eliminated, thereby making it possible to reduce thenumber of components significantly, and thus make a reduction in cost.

The present invention is not to be considered limited to the embodimentdescribed above, but various applications and modifications can be madewithin the scope of the invention.

DESCRIPTION OF REFERENCE SYMBOLS

10: battery element

11: positive electrode

12: negative electrode

13: separator

14, 15: lead tab

16, 17: sealant

18: outer package material

20: non-aqueous electrolyte secondary battery

1. A method for controlling a non-aqueous electrolyte secondary battery,the method comprising: connecting two non-aqueous electrolyte secondarybatteries in series; and setting a discharge cutoff voltage to 3.4 V to4.6 V.
 2. The method according to claim 1, wherein each of the twonon-aqueous electrolyte secondary batteries connected in seriescomprises: an exterior material; a positive electrode housed in theexterior material, the positive electrode including a lithium-transitionmetal oxide that has a layered crystal structure; a negative electrodehoused in the exterior material, the negative electrode including aspinel-type lithium-titanium composite oxide; and a non-aqueouselectrolyte in the exterior material, the positive electrode and thenegative electrode meeting 1.0>X, wherein X is an actual electriccapacity ratio denoted by (B/A), A is an actual electric capacity (mAh)at 25° C. per 1 cm² area of the positive electrode, and B is an actualelectric capacity (mAh) at 25° C. per 1 cm² area of the negativeelectrode.
 3. The method according to claim 1, further comprisingsetting a charge cutoff voltage to 5.50 V.
 4. The method according toclaim 1, further comprising setting a charge cutoff voltage to 5.20 V.5. The method according to claim 1, further comprising setting a chargecutoff voltage to 5.00 V.